dana bash takes us behind the scenes. >> reporter: no, that's not the president. it's a stand-in. but this is the kind of exhaustive prep going on to make sure there are no mistakes. it's not just for the president. senator chuck schumer is the events emcee. >> oh yeah. i don't want to do what chief justice roberts did. he's brilliant and he messed up. >> reporter: roberts messed up the oath four years ago. >> i'll execute -- >> faithfully the office of the president faithfully. >> reporter: he had to go to the white house for a do-over. who what are you afraid of? >> introducing the wrong person, who knows? >> reporter: as chair of the inaugural committee, assumer is preparing for this day more than a year. >> we didn't know who the president would be. but together, it's always done in a bipartisan way. >> reporter: a central focus, trying to avoid problems that put a damper on the last obama inauguration. >> tens of thousands of people missed it. they came in from all over the country. so, we've done symbolic things like get rid of the infamous purple tickets and there was a tun